Syntheses of Cyclic Poly(l‐lactide)s by Means of Zinc‐Based Ring‐Opening Polymerization with Simultaneous Polycondensation (ROPPOC) Catalysts

Abstract

Ring‐opening polymerizations of l‐lactide are studied in bulk at 140 or 160 °C with zinc n‐hexanoate, zinc 4‐chlorothiophenolate, and zinc pentafluoro thiophenolate (ZnSPF) as catalysts. The reactivity increases in the given order. With all three catalysts a high fraction of cycles is obtained only at polymerization (annealing) times around 7 d. With ZnSPF weight average molecular weights (Mw) up to 178 000, a Tm around 199 °C and a ΔHm around 99 J g−1 w ere achieved. The samples annealed for 4 or 7 d also display a saw tooth pattern of the mass peak distribution in the matrix‐assisted laser desorption/ionization time of flight spectra indicating transesterification reactions across the surface of extended ring crystals. This process optimizes the thermodynamical properties of the crystalline cyclic polylactides and is responsible for the high Tm and ΔHm values.

Transformation of poly(l-lactide) crystals composed of linear chains into crystals composed of cycles

“…The origin and significance of this unusual mass distribution have been intensively discussed. 23–27 To ensure a plausible and consistent interpretation of the present study, a brief summary of these previous results is necessary. It should be noted that STPs are unique to cyclic PLAs and have never been observed in linear PLAs.…”